Reliable measurement or estimation of various parameters of a motor vehicle powertrain is essential to proper electronic control of the powertrain components. For example, automatic shifting of a multi-gear ratio power transmission requires a reliable measure or estimation of the transmission input speed. While the essential parameters are often measured with dedicated sensors, the control must include provisions for operation of the powertrain components in the event of sensor malfunctions. For example, if a transmission input speed sensor malfunctions during operation of the vehicle, the powertrain controller can respond by reverting to a “limp-home” control mode in which the transmission only operates in a specified gear ratio. Alternatively, it may be possible for the powertrain controller to reliably determine a default value for the transmission input speed based on other sensor data and provide normal or near-normal control of the transmission based on the default value. For example, the transmission input speed may be calculated based on measured vehicle speed or on measured transmission output speed and gear ratio if gear shifting is not in progress. However, it can be difficult to determine if the measured vehicle or transmission output speed signals are reliable, and knowledge of the transmission input speed is usually required during gear shifting for purposes of shift quality control. Accordingly, what is needed is a method of determining a reliable and continuously available default value of transmission input speed for use in powertrain control in the event of one or more speed sensor malfunctions.